This invention relates generally to improvements in magnetic bearing structures. More particularly, the present invention relates to a magnetic bearing structure which utilizes a first bearing to radially flux couple a rotatable member to a stationary member, and a second bearing to axially flux couple the rotatable member to the stationary member in such a manner as to provide controlled radial, thrust and moment load support of the rotatable member relative to the stationary member.
Electromagnetic bearings are highly effective for supporting a body, such as a rotating shaft, which is effectively floated or levitated by magnetic fields. In this way the rotating shaft has no frictional contact with any stationary structure, thereby permitting relatively friction free rotation of the shaft or rotation of a body about the shaft. This arrangement possesses the obvious advantage that there is no mechanical abrasion, which results in reduced mechanical noise and durability not available with other types of bearing structures. Moreover, because of the reduced frictional effects which would otherwise be encountered with conventional bearing structures, it is possible to obtain higher speeds of rotation with electromagnetic bearings.
Magnetic bearings typically require little maintenance and readily lend themselves to operation in hostile environments such as in connection with corrosive fluids where other conventional bearings would be destroyed or rendered inoperable. Further, magnetic bearings are suitable for supporting moving bodies in a vacuum, such as in outer space, or in canned pumps where the pump rotor must be supported without the use of physically contacting bearings.
Conventional electromagnets utilized for energizing levitation gaps are inefficient in that they require a substantial amount of electrical power to generate the required electromagnetic field. In general, prior electromagnetic bearings require large electromagnetic coils and electronic-controlled circuitry which have been found to be inherently inefficient. There have been some proposals to use permanent magnets in combination with electromagnets in order to provide greater stabilization and control. However, the conventional prior designs, which utilize both electromagnets and permanent magnets, are inefficient from a spacial standpoint and are considerably complex.
One of the primary considerations in the development of magnetic bearing structures is to eliminate so-called air gaps. The so-called air gaps form a portion of the magnetic flux pathway of the electromagnets and permanent magnets, and provide a bridge between a supporting structure and a levitated structure. In actuality, some air gaps must be tolerated in order to position a suspended or rotatable body. Thus, air gaps to some extent cannot be avoided, but it is desirable to reduce air gaps to an absolute minimum.
From a pure physics standpoint, an air gap introduces great inefficiency into any type of magnetic structure. An air gap is about 2,000 times less efficient than an iron core medium for transmitting magnetic flux. Thus, in terms of inefficiency, a magnetic bearing structure which has an air gap of 0.1 inch is far more inefficient than a magnetic bearing which has an iron gap of 20 inches.
In addition, it is important to overcome the conductivity constraints of permanent magnets. Essentially, permanent magnets are very poor conductors for a magnetic flux, even though they generate magnetic flux. The most efficient permanent magnets available are the rare earth alloy magnets. Such permanent magnets, however, have a very low magnetic permeability and they behave in much the same manner as air gaps in the magnetic circuit. The low permeability of rare earth alloy magnets requires significant power to drive electromagnetic fields through the permanent magnets, thereby resulting in low electrical efficiencies. Thus, it is undesirable to transmit an electromagnetic field through a permanent magnet.
Moreover, in some working environments it is desirable to provide radial, thrust and moment load support to a shaft at or adjacent to one end of the shaft only, while permitting rotation of the shaft relative to a stationary housing. Such shaft support lends itself to gimballed mirror, gimballed sensor or gimballed optics configurations. Further, it is desirable to minimize the number of controls required for complete shaft support and control.
Accordingly, there has been a need for a novel electromagnetic bearing structure which utilizes a combination of radially polarized and axially polarized magnetic fields to produce a compact and spacially efficient structure which is light weight and obtains a high power efficiency. Additionally, there exists a need for an electromagnetic bearing structure wherein magnetic efficiency of the device is optimized b minimizing air gaps between the levitated and support structures, and wherein the electromagnetic coils are not required to provide magnetomotive forces to drive magnetic flux through permanent magnets. Further, such an electromagnetic bearing structure is needed which can utilize a permanent magnet bias to reduce power consumption to the controlling electromagnetic coils, and which lends itself to concurrent use of electromagnets and permanent magnets for the purpose of providing a high density, constant magnetic flux between associated structures. Moreover, an electromagnetic bearing structure is needed which is scalable, can exploit advances in permanent magnet technology, and can provide full radial, thrust and moment load support to a rotatable shaft at one end thereof. The present invention fulfills these needs and provides other related advantages.